In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, a developing roller is used for developing an electrostatic latent image formed on a surface of a photoreceptor body into a toner image
In general, the developing roller is produced by blending an electron conductive agent and/or an ion conductive agent with a rubber to prepare a semiconductive rubber composition imparted with a semiconductivity, forming the semiconductive rubber composition into a tubular body, and crosslinking the tubular body.
In recent years, the developing roller is required to have a reduced diameter for size reduction of the image forming apparatus, and the image forming apparatus is required to operate at a higher speed, so that the rotation speed of the developing roller tends to be increased.
Further, in recent years, a layer forming blade kept in abutment against the developing roller in a developing section of the image forming apparatus tends to apply an increased abutment force to an outer peripheral surface of the developing roller in order to improve the toner chargeability.
Therefore, parts of the outer peripheral surface of the developing roller against which opposite edge portions of the layer forming blade are kept in abutment are problematically liable to be worn or damaged in a short period of time, and the toner is often liable to leak laterally from the opposite edge portions of the layer forming blade.
Particularly, where the developing roller is allowed to have a porous structure to be imparted with an improved flexibility by foaming and crosslinking the semiconductive rubber composition with the use of a foaming agent, the problem of the lateral toner leakage tends to occur in a shorter period of time.
To cope with this, it is proposed to allow the developing roller to have a porous structure to be imparted with proper flexibility, and to form a coating layer on the outer peripheral surface of the developing roller by applying a coating agent containing a urethane resin, a phenol resin, a fluororesin, a silicone resin or the like to at least the parts of the outer peripheral surface of the developing roller to be kept in abutment against the opposite edge portions of the layer forming blade and drying the coating agent and, where the coating agent contains the urethane resin or the phenol resin, curing the resin (Patent Document 1 and the like).
However, the coating layer of the urethane resin or the phenol resin has a poorer slidability though being excellent in wear resistance. Therefore, the developing roller is liable to have an increased rotation torque, making it difficult to properly perform the image formation. On the other hand, the coating layer of the fluororesin or the silicone resin is excellent in slidability but has an insufficient wear resistance and, therefore, is liable to be worn to lose its effect in a shorter period of time.
For preparation of the coating agent, an organic solvent is required. The use of the organic solvent may exert a great load on the environment, and go against a recent trend toward reduction of VOC (volatile organic compounds).
It is contemplated to form a tubular porous inner layer of a foamed product of a semiconductive rubber composition and cover a generally entire outer peripheral surface of the inner layer with an outer layer of a seamless semiconductive resin tube (Patent Documents 2 and 3, and the like).
The tube for the outer layer is formed by extruding a resin. A resin excellent in strength, wear resistance and the like is selected for use as the resin for the tube, because there is no need to consider the solubility of the resin in an organic solvent. Therefore, the outer layer thus formed has a higher strength and an excellent wear resistance.
Further, the outer layer formed from the aforementioned resin through the aforementioned process has a smooth surface having an excellent slidability to suppress the increase in the rotation torque of the developing roller, making it possible to properly form an image.